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Appendix II.                                 825

:       The machine in Fig 795 was described by Mr. J. H> Gibson,'
*^ a paper read before the North-East Coast Institution in 1897.
.    is the driving shaft, connected to the cutter shaft B by change
Keels, arranged to give the actual relative motion between the
% <Orm and wheel.     The cutter D makes a purely circular rota-
v3^>n, which, however, traces a helical cut on the wheel blank N.
,   Ke shaft B is driven through nut E,- wheel F, and wheel G, the
riding on a feather key in the screw; but while cutting, all

pieces rotate solidly in the bearing H. Only once per
^volution of the wheel blank N, the lever p is pressed out by
cam Q, bringing rod R forward, as shewn by arrow, so that the
wheel j may catch R and cause wheel K to roll round G and F.
G has 42 teeth and F 40, these two wheels thus move relatively
one another, and the screw is shifted axially forward by the
pitch. The blank being fixed on the face plate, and the
p set just past the cam, the cutter is placed at L, and the
started. The result of the various motions is to make
^ series of light cuts or scratches on the blank, marking out the
l:r*terspaces ; but, when N has turned round once, the lever p
^oves, and, as previously explained, the cutter takes up a new-
Position on the helix of the imaginary worm. The cutting still
Proceeding automatically, the same grooves are simply cut a little
Deeper, and so the cycle of operations is repeated till the cutter
^merges at M, by which time every interspace will have been
Completely finished; for the cutter, representing the worm, will
have been presented in every one of its many positions, and
the wheel will have been truly 'rolled.' Finally the worm is
turned, with the cutter as template, and of a pitch equal to
screw BJ of which various sizes are kept.

/ P. 277, Standard Fits.  Mr. Arthur G. Fuller has re-
cently made very careful experiments to determine the proper
zt/orking fit clearance, which should vary with the size of the
abject. As regards driving fits > the pin must be slightly larger
tfoan the hole, but the amount depends on the strength of drive
required. Force fits require a still larger pin, but the size would
again depend on whether the force applied were that of a lever,
screw, or hydraulic press. The last-mentioned fits Mr. Fuller
estimated from the average of all the experience he could collect,